CHANGES IN THE LATER DEVONIAN 87 



Alleghenies of that state have been reached the mass has become dimin- 

 ished one-half, while in central western Pennsylvania the section is 

 ])arely one-fifth as thick as in Fulton county of the same state ; and in 

 Ohio the deposit becomes thin and recognizable only with difficulty. 

 Southward from New river the rate of subsidence decreased and the 

 trough became narrower. At 125 miles beyond that river the great mass 

 has become only a few hundred feet thick even on the edge of the Great 

 valley, and the deposits appear to reach not very far into Tennessee ; but 

 outlying areas show that the rapidly tapering trough continued into 

 northwestern Georgia and possibly into Alabama. 



The material of the Chemung deposits is fine grained throughout, with 

 the exception of two conglomerates, very persistent along the eastern 

 border to beyond New river in Virginia, but becoming somewhat finer 

 westward, where they are the first and third oil-sands of Pennsylvania. 

 The advent of the Catskill (of Vanuxem, Hampshire of Darton) was 

 marked by physical changes which gave to that formation a good claim 

 to recognition as a natural group. The area of sedimentation became 

 contracted so as to coincide rather closely with the Chemung area of 

 chief subsidence. Eastwardly it extended as far as did the Chemung, 

 but westwardly it reached not more than 30 miles beyond the line of 

 the Alleghenies of Pennsylvania, while southwardly it became shallower 

 to the shoreline, which lay apparently in Montgomery county of Vir- 

 ginia. Western Pennsylvania, eastern Ohio, almost the whole of Ken- 

 tucky, West Virginia, and Tennessee received no deposits, so that there 

 was land or waler too shallow to receive any deposit all the way from 

 the Cincinnati peninsula almost to the line of the Pennsylvania Alle- 

 ghenies. That the basin was wholly landlocked southward is not prob- 

 able, for the writer obtained Spirifer disjunctus from the upper beds near 

 Salem, Virginia, not very far from the last traces of Catskill toward the 

 south. Throughout the Catskill the deposit is fine grained, mostly mud 

 and muddy sandstones, with red and green as the prevailing colors. 

 Toward the close a conglomerate appeared at the northeast, which, how- 

 ever, extends but a short distance southwestwardly. 



The Chemung conditions were restored and exceeded at the close of the 

 Catskill, and deposits belonging to the later Devonian (Lower Pocono, 

 Cuyahoga, etcetera,) show that the area of sedimentation gradually en- 

 croached upon the land, west and south, so that they finally covered the 

 whole basin north from the Tennessee line, thus overlapping the Chemung 

 at the west and northwest. Southward from the Tennessee line the basin 

 contracted on the westerly side and very quickly was confined to the 

 area of eastern Tennessee and northwest Georgia where there seems to 

 have been a deep arm of the sea during the Chemung and latest De- 



XII— Bull. Geol. Soc. Am,, Vol. U, 1902 



